1. Field of the Invention
This invention relates to vehicle brake systems and more particularly, to a brake force control device for controlling the braking force imparted to the wheels of a vehicle.
2. Description of the Related Art
A conventional brake force control device for a vehicle is disclosed in Japanese Patent Laid Open No. 3(1991)-5270. A brake pressure circuit used in connection with a conventional brake force control device is shown in FIG. 6.
With reference to FIG. 6, the brake force control device for a vehicle comprises a master cylinder 100 operated by a brake pedal 101 to generate a brake pressure, a wheel brake 102 including a brake cylinder which brakes a wheel by the brake pressure, and a brake force control actuator 103. The brake force control actuator 103 includes a pressure control chamber connected to the brake cylinder and a displacement actuator which varies the capacity of the pressure control chamber.
The master cylinder 100 is connected to the wheel brake 102 by a conduit 104 having a first normally open solenoid valve 105 and a second normally open solenoid valve 106.
A first one way valve 107 which prevents brake fluid from flowing to the wheel brake 102 from the master cylinder 100 is provided in parallel arrangement with the first normally open solenoid valve 105. Further, a second one way valve 108 which prevents brake fluid from flowing to the master cylinder 100 from the wheel brake 102 is provided in parallel arrangement with the second normally open solenoid valve 106.
The displacement actuator installed in the brake force control actuator 103 is driven by an electric motor 103a to vary the capacity of the pressure control chamber. The first solenoid valve 105 is closed in order to separate the wheel brake 102 and the pressure control chamber from the master cylinder 100 when the wheel brake 102 is under an antilock condition. On the other hand, the second solenoid valve 106 is closed when the wheel brake 102 is under an antislip condition. At these times, the displacement actuator increases and decreases the capacity of the pressure control chamber. Therefore, the brake force control actuator 103 increases and decreases the brake pressure in the wheel brake 102 since the pressure control chamber is connected to the wheel brake 102.
Under the antislip condition, the brake fluid quickly flows into the wheel brake 102 from the master cylinder 100 through the second one way valve 108 when the vehicle needs to be stopped. Further, under the antilock condition, the brake fluid rapidly flows into the master cylinder 100 from the wheel brake 102 through the first one way valve 107 when the brake pedal is released.
In this known system, the wheel brake 102 is supplied with brake fluid only from the master cylinder 100.
Further, the frictional coefficient of the disc pad is designed to be low in order to prevent the disc pad from judderring. Therefore, a large amount of brake fluid needs to be used in the brake force control device because the area of the disc pad and the piston diameter of the disc brake must be increased so as to apply a brake force to a wheel effectively with the low frictional coefficient of the disc pad. Consequently, the brake force control device for a vehicle is expensive because of the required enlargement of the master cylinder diameter. Further, the brake force control device for a vehicle spoils the brake operation feeling because of the lengthened master cylinder stroke.